Link's Crossbow Training is to the Zapper what Wii Sports is to the Wii: a pack-in game for a piece of hardware. The only difference is that Wii Sports is a great game whereas Link's Crossbow Training is fairly average.
The game uses various environments from The Legend of Zelda: Twilight Princess as stages for targets with various background props that can be shot. It sports many characters, friendly and hostile, that play a role as targets or obstacles to avoid shooting, again all drawn from Twilight.
Even though the game has no bearing on the events in Twilight Princess you'll still be assuming the role of the protagonist of The Legend of Zelda series, the boy in green, Link. In order to perfect his crossbow marksmanship, you must pass a series of tests, starting with stationary bullseye targets, before moving onto moving targets and actual enemies. Forget Twilight Princess' story; that game merely serves as a backdrop for Crossbow Training.
Crossbow Training features 27 playable levels, and the goal in each is to achieve the highest score possible within the time limit. You'll find yourself finishing these missions in around an hour, though you could stretch that out to two if you're silly enough to want to get platinum scores by repeating levels.
If the price of other similar products is anything to go by, The Zapper is only really worth about $20, so the game itself is worth approximately $40. This isn't really good value for money, especially when you take into consideration that the Zapper is actually more of a hindrance than anything - playing naturally is actually a lot more fun, a lot less tiresome, and a whole lot more accurate.
It helps a little by removing the nunchuk from the Zapper setup and holding it in your left hand, but even so, the unit is still quite cumbersome when held this way. If you really want to use a 'Zapper' you're better off getting Futuretronics' Wii Rifle as it is considerably more comfortable.
The game looks and sounds like Zelda but truthfully is Zelda in name only. Don't expect to be charmed in the slightest. Over the years we've seen other Nintendo heroes fall in the endless pit of mini-games and ill-fated spin-offs, and seeing the rich world of Hyrule be perverted for such an average game is quite disconcerting. It plagues one with worry as to how Nintendo will milk the Zelda franchise further.
Fingers crossed that the next Zelda game will be a true Legend of Zelda and not another needless spin-off - it'll be a dark day for the entire industry if we see something like Zelda: Extreme Horse Riding come about.